The present invention relates to facsimile reproduction and particularly to methods for converting a video signal, generated by scanning an original copy dot by dot and line by line, into a bilevel black/white signal, and a circuit arrangement for carrying out the method.
In black/white facsimile reproduction, an original to be copied is scanned dot by dot and line by line in a scanner by means of an opto-electronic scanning member and the brightness information of the original is converted into a video signal.
The original may be a printed or type-written document, a hand-written text or a graphic representation in which both the background of the original and the information placed on it can be white, black or tinted (grey; coloured).
The video signal obtained by scanning the original is amplified, converted into a bilevel black/white signal by comparing it with a threshold signal and transmitted to a receiver via a transmission channel. The recording element of the receiver controlled by the video signal generates the required copy of the original.
When a white portion of the image of the original copy is being scanned the scanning member provides a high video signal amplitude, scanning a black portion of the image results in a low amplitude and a grey or coloured detail in the original in a medium amplitude of the video signal.
In order to generate the black/white signal, the different video signal amplitudes are continuously compared with the threshold signal to arrive at a decision if a video signal amplitude is to be evaluated as "white" or "black".
There are difficulties in correctly evaluating small variations in amplitude in the video signal caused by low contrast between the information and the background, often leading to losses of information in the copy.
U.S. Pat. No. 3,159,815 describes a method of comparing the video signal with a constant threshold signal for converting it into the black/white signal. With this so-called "constant threshold" evaluation is quite inadequate with a tinted original copy. If, for example, an original with a tinted background and black or white information content is scanned and the decision for medium amplitudes of the video signal is for "black" the black information on tinted background is lost; if, on the other hand, the decision is for "white" the white information on tinted background will not be considered.
German patent specification No. 1537560 describes a facsimile scanner with a threshold circuit in which the black/white signal is obtained by comparing the video signal with a threshold signal which follows it dynamically.
Although the so-called "dynamic threshold" is suitable for correct evaluation of small amplitude changes it will provide inadequate results if a white area in the original copy is followed in the direction of scanning by a tinted and a black area (white/grey/black transition) or a black area is followed by a tinted and a white area in the original copy (black/grey/white transition).
Since the scanning member recognizes both a grey and a coloured, that is a tinted, area in the original copy as "grey", for reasons of simplicity the text which follows will talk about transitions from and to "grey".
A typical example for such transitions is provided by a white-edged original copy with a tinted (grey or coloured) background containing black information (letters). The said threshold circuit interprets the tinted background following a white/grey transition as "black". In the copy, therefore, the front edge of the first black item of information is lost, that is a grey/black transition is ignored.
In addition, the threshold circuit interprets such a background area after a black/grey transition as "white". In the copy, therefore, the boundary between the background area and the white edge of the original copy is lost, that is a grey/white transition is also ignored.
The loss of information in the copy is considered to be a considerable disadvantage of the known threshold circuit.
Although it would be possible to adjust the customary threshold circuit to such a sensitivity that the background area is evaluated as "white" and the black information on it is recognized, information of low density such as, for example, red writing on a white surface would be lost. Thus individual adjustment will always only result in a compromise between high sensitivity and loss of information. The prerequisite for such adjustment is an experienced operator, and it is also time-consuming since test copies must be made for evaluation.
It is an object of the invention, therefore, to specify a method and a circuit arrangement in facsimile reproduction, for converting a video signal obtained by scanning an original copy dot by dot and line by line, into a bilevel black/white signal by means of which coloured original copies are converted to purely black/white copies without any significant loss of information. With these original copies both the background areas and the information placed on them can be tinted (coloured, grey), black or white.